Chronic Pain in Small Animal Medicine | PDF Download

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Over the past decade, since publication of the first edition of Chronic Pain in Small Animal Medicine, many advances have been made in the discipline of pain management, including embracement under the One Medicine initiative to improve the health and well-being of multiple species. Contributing significantly to this progress is the evidence base provided by multimodal management of chronic diseases such as osteoarthritis, a leading cause of pet euthanasia. These advances are explored in this updated edition, written for the veterinary professional seeking a greater depth of knowledge in the mechanisms of pain accompanying chronic disease states and the potential targets for treatment. Additional new sections describe newer drugs that are now in wide use, the Canine Osteoarthritis Staging Tool (COAST), novel approaches to cancer treatment, and cannabinoids and their functions.

The book goes beyond common protocols by focusing on the latest evidence and our understanding of ‘why and how to treat’. It describes and evaluates current physiological and biochemical theories of pain transmission without losing sight of the practical need for such information. Chronic Pain in Small Animal Medicine provides a foundation for advances in animal care and welfare and is necessary reading for veterinarians in practice and training.

We’re living in an age of exciting, new discoveries, but these are only exciting if we are aware of these offerings and their optimal indications for use. This book aims to open veterinarians’ eyes to the myriad new ways we can now treat chronic pain in small animals.

Preface

There is no pain pathway! Pain is the result of a complex signaling network. The cognition of pain, like cognition in general, requires sophisticated neurological hardware.

Pain has many definitions because it’s an intensely subjective experience that is filtered through our emotions as well as our anatomy. It’s any sensation amplified to an uncomfortable level, and it’s a plethora of negative emotions called ‘suffering’. No one patient feels pain the same – there is no single accepted pain experience. Like the perception of beauty, it’s very real, but only in the eye of the beholder. Yet pain is so fundamental to our well-being that it is added to heart rate, respiratory rate, temperature, and blood pressure as the ‘fifth vital sign’. Without a ‘pain thermometer’, people in pain must rely on their language skills to describe what they are feeling. In human medicine, pain is what the patient says it is; in veterinary medicine, pain is what the assessor says it is! Trained as scientists, veterinarians are schooled to assess responses based on the mean ± standard deviation, yet effective pain management suggests we target the leastrespondent patient within the population, so as to ensure no patient is declined the relief of pain it needs and deserves.

In its simplest sense, pain protects us from bodily harm; hence the proposal that pain is a teacher, the headmaster of nature’s survival school. Dangerous things are noxious things, and pain punishes us if we take excessive risks or push ourselves beyond our physical limits. Further, pain often forces us to observe ‘recovery time’. Another way of understanding pain is that any stimulus – noxious or otherwise – can become painful if the patient’s ability to cope with it has been diminished.

A working definition of chronic pain is that, unlike acute pain, it lasts beyond the time necessary for healing and resists normal treatment. The primary indicator of chronic pain is not how long it persists, but whether it remains long after it should have disappeared. As the father of pain medicine, John Bonica, explains, ‘Acute pain is a symptom of disease; chronic pain itself is a disease’.

The noxious stimuli that constitute pain can reconfigure the architecture of the nervous system they invade. Lasting noxious input can produce a neurobiological cycle of chemical and electrical action and reactions that becomes an automatic feedback loop: a chronic, self-perpetuating torment that persists long after the original trauma has healed.

From the human healthcare experience, pain, and in particular chronic pain, is a major problem for which current treatments are often inadequate. The tangible costs economically are in the many tens of billions of dollars, and the costs in terms of suffering are known all too well to practitioners who seek to help these patients. In veterinary medicine we are experiencing a surge of increasing focus on measuring and resolving pain and suffering, and indeed, this aspect is central to the veterinarian’s oath. This focus is being supported by an increased understanding of pain neurophysiology, discovery of novel treatment targets, a greater offering of innovative pharmacologics, and consumer demand. The pharmaceutical industry has made important strides forward in bringing new therapies to address the problem of chronic pain, but to the suffering patient, this progress is glacially slow. Specific areas of exploration include peripheral nervous system targets, central nervous system targets, disease-specific targets, and development of measurement tools and applications of new technologies.

In the 1880s, Friedrich Bayer and Company commercialized Bayer Aspirin. When aspirin (‘a’ for acetyl, part of its chemical composition; ‘spir’ from a plant that contained salicin; and ‘in’, a popular medical suffix at the time) went over the counter in 1915, the mass production of pain alleviators for the general public was launched. Pain is the most common reason patients see a physician, while pain and pain relief are among the most robust areas of medical research. 

Realistically, new discoveries and innovative drug formulations for veterinary patients will continue to lag considerably behind those for humans, despite the fact that animals are often used for the development of human therapies. This is a reality of present-day economics, appreciated as return on investment by the pharmaceutical industry. Accordingly, there are presently, and will likely in the future be, a limited number of agents and techniques actually labeled for veterinary use. It is therefore incumbent on the veterinarian and veterinary healthcare professional to understand both the neurobiology of chronic pain and the mode of action of various therapies so as to determine if the therapeutic agent or technique is likely to be safe and efficacious when utilized ‘off-label’. Such insights may not be readily available for the proposed target patient, but would be ‘inferred’ from data obtained from a different species. Herein comes the weighing of ‘species specificity’ vs. ‘one science’ in the clinical decision-making process.

So . . . what’s new? Medicine, in general, will always engender creativity and innovation, some of which challenges our dogmatic propensity. For example, in a surprising discovery that flies in the face of conventional medicine, McGill University researchers report that treating pain with anti-inflammatory medications, like ibuprofen or aspirin, may promote pain in the long term. Published in Science Translational Medicine, the authors suggest that inflammation, a normal part of injury recovery, helps resolve acute pain and prevents it from becoming chronic (from Emily Shiffer. How We Treat Acute Pain Could Be Wrong. Medscape: 17 June 2022). It might be proposed that blocking that inflammation may interfere with this process, leading to harder-to-treat pain. Further, “What we’ve been doing for decades not only appears to be mis-guided but is 180 degrees out. We should not be blocking inflammation. We should allow inflammation to play its role: nature’s way of obtunding chronic pain”. Testing this concept, researchers blocked neutrophils in mice, finding the pain lasted 2 to 10 times longer than normal. Antiinflammatory drugs, despite providing shortterm relief, had the same pain-prolonging effect, though injecting neutrophils into the mice seemed to keep that from happening. Inflammation occurs for a reason, and it appears that it may be dangerous to interfere with it. Does this mean taking an analgesic, like acetaminophen (paracetamol), without blocking neutrophils may be better than taking an anti-inflammatory drug or steroid? Stay tuned for the third edition, wherein we may have the answer.

This text was created for the veterinary healthcare professional seeking a greater depth of knowledge in mechanisms of pain accompanying chronic disease states and potential targets for treatment. It aspires to go beyond the ‘cookbook protocols’ found in many offerings by providing contemporary understandings of ‘why and how to treat’.

An attention-getting photo is that of the number of scientific medical publications in the past couple of years placed adjacent to the number of scientific medical publications in the previous couple of decades. The former intimidates the latter – the point being that new information is drowning us like a tsunami, not only in volume but also in breadth. To offer our patients the ‘best care’, providers need to be current in the knowledge of the ‘best medicine’. This requires a trilogy of synergism among researchers, publishers, and clinicians. Herein publications such as this require occasional revisions. This second edition attempts to meet segments of that challenge.

Several new therapeutics are introduced as well as combination products. Also, new delivery systems are revealed together with their effectiveness and safety, together with drivers for development. New discoveries regarding the role of macrophages is presented, particularly as related to osteoarthritis.

There is a new section on the use of radiosynoviorthesis as well as anti–nerve growth factor (NGF) monoclonal antibodies. This second edition also contains a section on recognition and assessment of pain in different species and reveals a staged approach to the treatment of osteoarthritis via the COAST scheme.

Additionally, commentary on acupuncture and cannabinoids is included; however, the topics of product selection, dosing, and treatment protocols have intentionally been avoided due to the contentions of wide variability from which consensus is lacking – topics for a different forum.

The first edition (2010) of this text has been translated into several different languages and has sold more than 850 copies – testimonial to the increasing focus on chronic pain management in veterinary medicine. I propose this can be attributed to several factors: (1) increases in pet owner demand, (2) advances in scientific discoveries, (3) economic entrepreneurship, and (4) (most significantly) the increased representation of female veterinarians. I personally feel that women are more empathetic to pain and are synergistic to the scientific advances in driving patient relief from pain.

I would like to share an observation before closing, an observation credible only from those of us active in clinical veterinary medicine over the past few decades.

Brakke Consulting estimates that of veterinary-approved NSAIDs (the most common scripted analgesic for dogs and cats) from Pain Management Product reports, U.S. sales figures at the manufacturer level are: Four decades ago, only 10% of graduating veterinarians were women. With the passing of Title IX and the removal of gender bias in veterinary college admissions during the 1970s, approximately four out of five veterinary students are now female. The number of female veterinarians now exceeds the number of male veterinarians (American Veterinary Medical Association [AVMA] members). In companion animal practice, the number of women now exceed men by more than 30%.

The increased use of analgesics in veterinary medicine certainly has a number of contributing factors; however, I propose that the increased use of analgesics and increased percentage of female veterinarians are more than coincidental. I believe there is an element of causality. It is my personal belief that, more often than not, women bring to veterinary medicine a greater empathy for pain than do males. In such recognition, I believe women should be proud of this significant contribution they have brought to our collaborative profession.